1. Field of the Invention
The present invention relates to optical sensing systems and more particularly, but not specifically to operation of such systems including an interferometer.
2. Description of Related Art
An optical sensing system may be constructed using an interferometer. The interferometer can comprise several lengths of optical fibre formed contiguously with partially reflecting discontinuities therebetween. Light pulses are introduced into the contiguous length of optical fibres such that a portion of each of these light pulses is reflected back along the contiguous length at each discontinuity. By suitable timing of the light pulses, interaction of reflected light pulses from respective successive partially reflective discontinuities can be achieved. This light pulse interaction is arranged to provide a composite heterodyne signal indicative of the optical fibre length or sensor constituted between the successive partially reflective discontinuities.
A typical approach taken to provide a composite heterodyne signal is to inject pairs of light pulses in timed succession, of specific pulse length and at mutually displaced or different frequencies. Thus, for example, there is a first light pulse F.sub.1 and a second light pulse F.sub.2. It will be appreciated that each light pulse F.sub.1 and F.sub.2 is reflected at each discontinuity. The timed succession of pulses F.sub.1, F.sub.2 is such that the partially reflected portions of pulse F.sub.1 interact with the partially reflected portions of pulse F.sub.2 to provide the composite heterodyne signal. If pulse F.sub.1 precedes pulse F.sub.2 upon injection into the contiguous length of optical fibre lengths then the partially reflected portion of pulse F.sub.1 interacts with a partially reflected pulse of F.sub.2 reflected by the immediately preceding partially reflective discontinuity.
Consider FIG. 1a and FIG. 1b, which illustrates respectively a prior sensing system and its timing diagram, wherein a pair of light pulses F.sub.1, F.sub.2 are injected regularly into the contiguous length of optical fibres with a time period 1 between each pair of respective pulses F.sub.1, F.sub.2. The light pulses F.sub.1, F.sub.2 are reflected by discontinuities DL, S1, S2, S3, S4, S5, . . . , SN, where N is an integral number, to provide partially reflected pulses F.sub.1 DL, F.sub.2 DL, F.sub.1 SN and F.sub.2 SN. The first partially reflective discontinuity DL is the "dead" or down lead of the sensing system. The partially reflective pulses F.sub.1 S1' and F.sub.2 DL' i.e. the light pulse portions reflected by discontinuities S1 and D1 for pulses F.sub.1 and F.sub.2 respectively, interact to provide a heterodyne (F.sub.1 S1'-F.sub.2 DL'). The heterodyne (F.sub.1 S1'-F.sub.1 DL') is indicative of the optical fibre length or sensor between discontinuities DL and S1 and is adaptive to changes therein. Thus, any change in the optical fibre sensor between discontinuities DL and S1 will be apparent in the heterodyne (F.sub.1 S1'-F.sub.2 DL'). Similarly partially reflected pulses for other discontinuities (S1, S2, S3, S4 and S5) will interact to give interactions indicative of their respective optical fibre lengths or sensors.
As will be appreciated it is important that all partially reflected pulses F.sub.1 DL', F.sub.2 DL', F.sub.1 SN and F.sub.2 SN have exited the contiguous length of optical fibre lengths before a next pair of light pulses can be injected into the contiguous length in order that pulse pairs from different sensor lengths do not overlap. Consequently, the pulse repetition rate or frequency is determined by the number of sensor lengths i.e. the more sensor lengths, the lower the pulse pair repetition rate.
In FIG. 2, a typical composite heterodyne signal 21 output is illustrated. The heterodyne signal 21 comprises a peak heterodyne frequency 23 with switching spikes 25 regularly spaced by increments of a pulse repetition frequency 27. It is the heterodyne frequency 23 which carries the information consequently the switching spikes 25 must be filtered out by suitably narrow filtering means. This filtering means in turn limits the maximum detectable modulation frequency of the sensing system. Thus, for example, an array of one hundred 200 meter long sensors will be limited to a maximum detectable modulation frequency of 1 KHz or less due to the low pulse repetition frequency allowed. However, if there is a requirement to detect acoustic frequencies up to 10 KHz, this reduces the allowable number of sensors to an inconvenient value of 10. The problem is to devise a system whereby the pulse pair repetition rate or frequency, and hence the maximum detectable modulation frequency, can be increased without reducing the number of sensor lengths.
Previously, the above problem has been ameliorated by decreasing pulse lengths and sensor length accordingly, however this reduces sensor sensitivity.
It is an objective of the present invention to provide a sensing system in which the detectable modulation frequency is enhanced with respect to the number of contiguous sensor lengths.